Ophthalmological operations on the cornea of the eye require exact knowledge of the position of the eye. There are principally two approaches for this purpose. First, the cornea of the eye can be spatially fixed by pressing a planar contact element onto it. This approach has the advantage that an exact alignment of the eye to the surgical instrument is possible. Disadvantages are an increase in intraocular pressure and a certain inconvenience for the patient.
Another approach aims to detect and adjust for movements of the eye during an operation. This approach also known as “tracking” has been realized in many ways in the prior art. Thus, for example, U.S. Pat. No. 6,280,436 or U.S. Pat. No. 5,481,622 describe the observation of the eye using a video camera. The position of the pupil is determined on the basis of the video image and an eye movement is derived from shifts in position. A similar approach is taken by WO 95/27454 which discloses a contact element wherein the contact element, provided as a metal ring, replaces the function of the pupil, i.e. a previously known biometry of the contact element is evaluated during video observation for detection of movements. Similar tracking concepts are known from U.S. Pat. Nos. 6,367,291, 6,283,954 and 6,210,401, which either evaluate the transition between the iris and the sclera for detection of movements or which monitor reference marks on a contact glass serving for eye fixation with an independent observation system.
DE 100 14 479 further suggests monitoring the iris of the eye by means of a camera, in order to enable detection of eye movements by an image evaluation method. Another possibility is to measure the distance from the device to several locations on the cornea. For this purpose, U.S. Pat. No. 6,315,773 uses a laser interferometer.
Tracking methods are in demand, in particular, in laser-surgical instruments, because they advantageously allow working without fixing the eye. In such laser-surgical methods, the treatment laser radiation is focused in or on the tissue so as to cause an optical breakthrough. The treatment laser radiation acts through photodisruption or photoablation.
In the tissue, several processes occur one after the other in time, which are initiated by the treatment laser radiation. If the power density of the radiation exceeds a threshold value, an optical breakthrough occurs, generating a plasma bubble in the tissue. This plasma bubble grows due to expanding gases after the optical breakthrough has formed. If the optical breakthrough is not maintained, the gas generated in the plasma bubble is absorbed by the surrounding tissue and the bubble disappears again. However, this process lasts considerably much longer than the generation of the bubble itself. If a plasma is generated at a tissue interface, which may just as well be located within a tissue structure, tissue is removed from said interface. This is then referred to as photoablation. In the case of a plasma bubble which separates previously connected tissue layers reference is usually made to photodisruption. For the sake of simplicity, all such processes are summarised here by the term optical breakthrough, i.e. this term includes not only the actual optical breakthrough, but also the effects in the tissue resulting therefrom.
It is indispensable for high precision of a laser-surgical method to ensure high localisation of the effect of the treatment laser beams and to avoid, if possible, collateral damage in adjacent tissue. Therefore, it is common in the prior art to apply the treatment laser radiation in a pulsed form, so that the threshold value for the power density of the treatment laser radiation, which is required in order to cause an optical breakthrough, is exceeded only during the individual pulses. High focusing of the laser beam in combination with very short pulses allows the optical breakthrough to be inserted in a tissue in a very punctiform manner.
The use of pulsed treatment laser radiation has become established recently, in particular, for laser-surgical correction of visual deficiencies in ophthalmology. Visual deficiencies of the eye are often due to the fact that the refractive properties of the cornea and of the lense do not cause proper focusing on the retina. In near-sightedness (also referred to as myopia), the focus of the relaxed eye is located in front of the retina, whereas in far-sightedness (also referred to as hyperopia) the focus is located behind the retina.
U.S. Pat. No. 5,984,916 as well as U.S. Pat. No. 6,110,166 describe methods for correction of visual deficiencies by suitably generating optical breakthroughs so as to ultimately influence the refractive properties of the cornea in a selective manner. A multiplicity of optical breakthroughs are placed next to each other such that a lense-shaped partial volume is isolated within the cornea of the eye. The lense-shaped partial volume separated from the remaining corneal tissue is then removed from the cornea by means of a laterally opening cut. The shape of the partial volume is selected such that, after removal, the refractive properties of the cornea are changed so as to cause the desired correction of the visual deficiency.
Of course, in order to isolate the partial volume, it is indispensable to generate the optical breakthroughs at predetermined locations. Uncontrolled eye movements would understandably result in the optical breakthroughs not being generated at the predetermined locations. The aforementioned detection of the eye movement during an operation is thus indispensable for non-contacting laser-surgical methods.
With regard to resolution, the known possibility can be realized in a limited or insufficient or very complex manner only. Moreover only very few approaches allow detection of a rotation of the eye about the optical axis. Therefore, it is an object of the invention to enable improved detection of eye movements with reduced complexity.